The invention relates to manufacturing glass fiber preforms, and in particular optical fiber preforms, and it relates in particular to a method and to apparatus making it possible specifically to increase the rate at which such preforms are manufactured.
Document FR-A-2 253 723 describes a method of preparing a preform for an optical waveguide in which a plasma torch projects glass powder which is injected into the base of the flame by a carrier gas. A flow of carrier gas is created so as to move the glass powder along a substantially horizontal elongate path. More precisely, in the absence of carrier gas, the glass powder does not advance towards the plasma torch so the plasma flame does not contain glass powder. That constitutes a method of pneumatically injecting glass powder.
Also known, from document GB-A-2 134 896 is a method of manufacturing a preform which is similar to the method of the first-mentioned document. A mechanically measured quantity of silica glass powder is inserted into a central tube into which there are inserted in succession oxygen and hydrogen to form an oxygen-hydrogen torch flame, so that the powder is inserted into the torch before a flame is formed. That method therefore includes injecting glass powder upstream from the base of the flame and via the inside of the flame.
Document FR-A-2 446 264 describes a method of preparing a preform for an optical waveguide in which a circular section cylindrical preform is moved along its axis past the flame of a plasma torch, said flame extending horizontally. A feed duct having a sloping bottom length inserts silica glass powder under the action of gravity to the top of the flame so that the powder is entrained by the flame onto the preform. However, although that document indicates that the plasma torch can be oriented in several ways in a vertical plane perpendicular to the axis of rotation of the preform, the description states that the grains of glass xe2x80x9cfallxe2x80x9d into the flame of the plasma, which means that the plasma flame must have a significant component in a horizontal direction.
The disposition described in that document FR-A-2 446 264 is simpler than the dispositions described in the previous documents because the operations of conveying the glass powder and of forming the plasma flame are completely separated and because glass powder is fed solely under the action of gravity.
In general terms, the invention relates to a method of the type described in above-mentioned document FR-A-2 446 264 in that it implements a plasma flame that is directed towards a preform, together with a separate device for feeding powder under gravity. Nevertheless, in the method of the invention, the glass powder gravity feed normally takes place only at the edge of the plasma flame. In such a method, the quantity of glass powder inserted into the plasma and deposited on the primary preform is limited by the depth to which the powder penetrates into the plasma flame.
According to the invention, the drawback due to the limitation of powder penetration depth into the plasma flame is eliminated or at least greatly reduced by making available a method and apparatus in which the rate at which glass preforms are manufactured in a system using gravity feed and a plasma flame is increased by reinforcing the gravity feed with acceleration of the glass powder before it penetrates into the flame. This acceleration is obtained by using a gas for acceleration the powder in the terminal portion of the feed device.
More precisely, the invention provides a method of manufacturing glass fiber preforms, and in particular optical fiber preforms, the method being of the type which comprises rotating a cylindrical primary preform about its axis; moving a plasma torch in translation relative to the preform in a direction parallel to the axis of the preform, the axis of the plasma torch flame not intersecting the axis of the preform and being offset by a certain distance from the axis of the preform; and inserting glass powder by gravity into the plasma flame from outside the flame; according to the invention, the method comprises accelerating the glass powder inserted under gravity before it penetrates into the plasma flame by means of an accelerator gas inserted into the flow of glass powder advancing under gravity; and adjusting the offset distance between the axis of the plasma flame and the axis of the preform so that the offset distance between the axes is decreased as the acceleration of the powder is increased.
Preferably, the acceleration is adjusted by adjusting the flow rate of the accelerator gas to a value that is smaller than or equal to 15 l/min, with the glass powder feed rate possibly being as much as 90 g/min.
Advantageously, the method comprises, prior to insertion of the accelerator gas into the flow of glass powder, preparatory treatment of the accelerator gas. This preparatory treatment of the accelerator gas can be constituted, in particular, by preheating the accelerator gas, and/or by inserting doping material into the accelerator gas.
The invention also provides a method of manufacturing glass fiber preforms, the method being of the type comprising a plasma torch, a device for imparting relative displacement in translation between a primary preform and the flame of the plasma torch, and a device for feeding the plasma flame under gravity with glass powder from outside said flame; according to the invention, the gravity feed device includes a bottom length of a tube having an accelerator gas injection tube connected upstream therefrom.
Preferably, the axis of the accelerator gas injection tube and the axis of the bottom length of the feed device form an angle that is less than or equal to 30xc2x0.
Advantageously, the apparatus further includes a device for adjusting the accelerator gas flow rate.
In an advantageous embodiment, the apparatus further includes a device for performing preparatory treatment to the accelerator gas prior to the tube for injecting the accelerator gas into the flow of glass powder. Advantageously, the device for preparatory treatment of the accelerator gas is selected from the group comprising a device for preheating the accelerator gas and a device for inserting a doping material into the accelerator gas. Preferably, the accelerator gas is air.
In an advantageous embodiment, the apparatus further comprises a device for adjusting the distance between the axis of the plasma flame of the torch and the axis of a primary preform on which the glass fiber preform is formed.